Follow my lunch: from bite to muscle
Ride one bite of sandwich through your gut, into your blood, and out to a working leg muscle.
A bite of lunch
This is your lunch: a bite of a cheese sandwich, mostly bread. Bread is full of starch, a long chain of sugar joined together. Your body cannot use starch as it is, so it has to take it apart and let the pieces into your blood. The glowing dot is a marker to follow, not a real bit of food: it stands for whatever part of the mouthful we are watching. Several metres of tube are folded up inside you, and your lunch is about to travel the whole way.
A cheese sandwich is mainly carbohydrate (starch from the bread), with fat and protein from the cheese. Starch is a polysaccharide: hundreds to thousands of glucose units joined by glycosidic bonds, too large to cross the gut wall. Digestion breaks it back down to glucose, and the protein to amino acids and the fat to fatty acids, so the body can absorb them. The marker is an illustration, not a real particle: a glucose molecule is about 0.9 nm across, far too small to draw beside a 1.7 m body. The gut shown is real Z-Anatomy scan mesh; the dot is not.
Chew and swallow
Your teeth mash the sandwich and your spit makes it soft and slippery. The spit even starts to break down the bread. You swallow, a little flap closes over your windpipe, and wave after wave of muscle squeezes the mouthful down your food pipe to your stomach. So far it is still food, not nutrients.
Chewing increases the surface area and salivary amylase begins breaking starch into maltose in the mouth. Swallowing moves the bolus past the epiglottis into the oesophagus, and peristaltic waves carry it to the stomach in a few seconds. Our marker here still stands for the whole chewed mouthful, not yet for single nutrient molecules.
The mixing bag
The stomach is a stretchy bag with strong muscle walls. It churns your lunch with acid and powerful juices until it turns into a thick soup, then lets the soup out a little at a time into the tube below. It is still food, just soup now.
The stomach stores and churns the bolus with gastric juice (hydrochloric acid, pH about 1.5 to 3.5, and the enzyme pepsin) to make chyme. Protein digestion starts here while starch carries on. The pyloric sphincter releases chyme into the duodenum in small squirts; a meal takes about 2 to 4 hours to leave.
Chopped into nutrients
The liver makes a green juice called bile, stored in the gallbladder, and the pancreas makes a juice full of tiny tools called enzymes. They pour onto the soup just past the stomach. The enzymes snip the long starch chain into single sugars called glucose. That is the moment food becomes nutrients: small molecules your body can soak up and use.
In the duodenum, pancreatic amylase breaks the rest of the starch into maltose, which enzymes on the gut lining (maltase) split into glucose; lipase splits fats, and proteases split proteins into amino acids; bile emulsifies fat into droplets so the enzymes can reach it. By the end of the small intestine the mouthful is a mix of single nutrients: glucose, amino acids, fatty acids, plus vitamins and minerals. Food is the complex material; nutrients are the small molecules it is reduced to.
Soaked up into the blood
The small intestine is lined with millions of tiny fingers that reach into the soup and soak up the nutrients. Watch the marker change here: a golden dot meant food, and now a green dot means a nutrient, glucose. That colour change is only a way of showing the change of meaning. In the body, millions of separate sugar molecules slip through the wall into the blood, each far too small to see. This is absorption. The sugary blood does not go straight to your heart: it goes first to the liver.
Folded villi and microvilli give the small intestine a huge absorptive surface. Glucose and amino acids cross the lining into blood capillaries inside each villus, by active transport and diffusion; most fats are repackaged into chylomicrons that enter the lymph instead. The water-soluble nutrients drain into the hepatic portal vein, which carries them to the liver before the rest of the body. The green dot is a marker for one nutrient, not a single mouthful travelling whole, and not a real object: real capillaries here are a few micrometres wide and are not modelled.
The liver checks the delivery
The nutrient-rich blood carries your sugar into the liver through a special vein called the portal vein. The liver is the body’s warehouse and inspector: it stores some sugar for later, turns some into other things it needs, and removes anything harmful. What is not needed right now is sent on through the big vein to the heart.
The hepatic portal vein delivers nutrient-rich blood to the liver, which takes up glucose, stores it as glycogen and releases it between meals, processes amino acids and detoxifies many substances (first-pass metabolism). Blood leaves the liver through the hepatic veins into the inferior vena cava and so to the right atrium. The liver holds roughly a tenth of the body’s blood at any moment. The liver’s inside is not modelled, so the dot fades into it and out again.
A detour for oxygen
The blood carrying your sugar reaches the right side of the heart, which pumps it to the lungs. The sugar does not need the lungs: the blood does. It stops to drop off waste gas and pick up fresh oxygen, then comes back to the left side of the heart, ready to be pumped out to the body.
From the right atrium the blood passes to the right ventricle and through the pulmonary arteries into the lungs, where it releases carbon dioxide and takes up oxygen. It returns through the pulmonary veins to the left atrium and left ventricle. The glucose stays in the blood throughout; only the gases change. This loop is the pulmonary circuit; the body loop is the systemic circuit.
Fuel for a working muscle
Now the left side of the heart pumps the blood out into the biggest pipe in the body, the aorta. Our marker rides down to the leg and into a thigh muscle that is hard at work: the rectus femoris. There, the oxygen and the glucose go into the muscle cells and are burned together to give the muscle energy to pull. That energy is why you can walk, run and lift things. The sugar and oxygen meet in capillaries far too small to model, so this last stretch is a stand-in.
Oxygen-rich blood leaves the left ventricle into the aorta, which divides into the common iliac and then the femoral artery, running down the thigh beside the rectus femoris. In the muscle capillaries glucose and oxygen cross into the muscle fibres. In the cell, glucose is broken down to pyruvate (glycolysis) and then, with oxygen, all the way to carbon dioxide and water inside the mitochondria, releasing ATP, the cell’s energy currency. Working muscle needs far more blood, so its arterioles widen and the heart beats faster. The capillaries themselves are not modelled; the dot’s last hop is illustrative.
The whole journey
Watch the whole journey at once. The golden dot follows your lunch from mouth to gut, where it is chipped into nutrients and the dot turns green. It is soaked up into the blood, passes the liver, is pumped through the heart and lungs to pick up oxygen, and at last reaches the working leg muscle, where the sugar is burned for energy. Eating to moving, one connected road. Use Ride along to travel it yourself.
mouth → oesophagus → stomach → small intestine (digestion and absorption) → hepatic portal vein → liver → hepatic veins → inferior vena cava → right atrium → right ventricle → pulmonary arteries → lungs → pulmonary veins → left atrium → left ventricle → aorta → common iliac artery → femoral artery → muscle capillaries. The whole gut trip takes 24 to 72 hours, but this sugar was absorbed within a few hours of eating, and once in the blood it reaches the muscle in around a minute. The 30 s loop shown is slowed down so you can watch it.